(1) Field of the Invention
The present invention relates to a magnetic toner for developing an electrostatically charged image. More particularly, the present invention relates to a magnetic toner for developing an electrostatically charged image, having an improved humidity dependency, which does not cause scattering or fogging even under a high humidity condition and which provides a sufficient image density with a high transfer efficiency.
(2) Description of the Prior Art
A so-called one-component type magnetic developer comprising a powder of a ferromagnetic material incorporated in developer particles is widely known and used as the developer capable of developing an electrostatically charged latent image without using a particular carrier.
When an electrostatic latent image is developed by the one-component type magnetic developer, individual developer particles simultaneously undergo an electrostatic attracting force (Coulomb force) of attracting the developer particles to the electrostatic latent image and a magnetic attracting force of attracting the developer particles to a magnet for forming a magnetic brush. The particles on which the Coulomb force imposed is larger than the magnetic force are attracted to the electrostatic latent image, while the particles on which the magnetic force imposed is larger than the Coulomb force are left on the developing sleeve, whereby development is effected according to the static latent image on a substrate.
The one-component type developer is divided into two types, that is, one type comprising a so-called electroconductive magnetic toner and a so-called insulating magnetic toner. These magnetic toners are ordinarily prepared according to the spray granulation method comprising dissolving or dispersing a magnetic material and a binder medium into a volatile solvent and spraying the solution or dispersion into drying air to effect granulation, or the pulverization method comprising comprising melt-kneading a magnetic material such as magnetite with a binder medium, cooling the melt and pulverizing the solidified mixture.
In these magnetic toners, a part of the magnetic material is exposed to the surface of the binder medium unless a resin having a high affinity with magnetite and being capable of forming magnetite into a film is used in case of the spray granulation method or unless a heat treatment is carried out after the pulverization in case of the pulverization method. Accordingly, if such special means is not adopted, these magnetic toners are readily influenced by water contained in air.
More specifically, in case of the electroconductive magnetic toner, under a high humidity condition, the toner particles are aggregated and are readily scattered, resulting in density unevenness such as fogging, and in case of the insulating magnetic toner, since the charging capacity of the toner per se is small and since the charge quantity is small and the charge retaining property is poor when the toner is charged, in a high-humidity atmosphere, the charge is readily lost by leaking and therefore, the density of the developed toner image is reduced, a sharp image can hardly be obtained and the transfer efficiency is reduced.